It is customary to check the oil level in a piston engine aircraft engine prior to takeoff. This involves opening a hatch door on the engine cowling, removing the dipstick, checking the oil level on the dip stick, and adding oil as required. While adding oil, the pilot (or mechanic) cannot return the dipstick to its appropriate location (hole) in the engine, since the dipstick hole is used for adding oil. Therefore, pilots have been known to set the dipstick aside while adding the oil, and subsequently forget to replace the dipstick into the engine. This of course results in oil escaping from the engine during flight, and consequent emergency situations.